Surface failure of dielectric materials (i.e., electrical insulators) in the presence of contaminants is a phenomenon well known in the art. While electrical insulators may be fabricated having a very high resistance to electrical breakdown through a body of the dielectric material, a more likely mode of failure of such insulators is often over an outside surface of the insulator especially where the surface of the dielectric material of the insulator is exposed to environmental factors (e.g., rain mixed with air pollution). Where an insulator is exposed to such conditions, an electrical short circuit may occur across the surface of the insulator. Apart from the damage a short circuit may cause to the electrical system itself, the arc may also damage the insulator.
The conditions which may lead to arcing of an insulator in the presence of contaminants is often controlled by a number of factors. Chief among the factors is the voltage applied across the insulator and the electrical rating of the insulator itself. Rain striking the insulator forms a thin conductive film across the surface of the insulator. Where the surface of the insulator is impervious to the penetration of water (e.g., fired ceramic) the film may remain thin and the potential for arcing small. Where the insulator is porous (e.g., unfired ceramic) the film of water and contaminants effectively becomes thicker due to penetration of the water and contaminants into the insulator.
When an arc occurs across a ceramic, the heat of the arc may drive away the water leaving only the undissolved contaminants of the air pollution deposited on the insulator as a solid coating material. Subsequent raindrops striking a fired ceramic insulator may dilute and wash away the deposited air pollution. On the other hand, where the insulator is fabricated of a porous ceramic, the contaminants are often deposited within the porous matrix of the ceramic and cannot be washed away. Subsequent deposits and arcing causes an accumulation of contaminants within and near the surface of the insulator and progressively more severe arcing. Where the contaminant is an organic material the heat of the arc will often result in the contaminant being reduced to a conductive carbonaceous material.
In either case, the localized presence of carbon on or near the surface of the insulator will often be the source of subsequent arcs and eventually a path of carbon will develop providing a track for subsequent arcing. The accumulation of the carbon into a continuous path is commonly referred to as tracking.
Where the insulator is also fabricated of an organic material (e.g., plastic), tracking can occur at a much faster rate due to thermal breakdown of the organic material. When a first arc occurs, the contaminants and also the plastic along the path of the arc may be converted into carbon by the heat of the arc. Since porosity of the insulator is an important factor in tracking, even non-electrolytes (e.g., solvents or airborne dust) may exacerbate tracking by accumulating and penetrating the insulator to provide a source of an initial and subsequent arcs.
In an effort to provide a measure of susceptibility of insulators to tracking, the International Electrotechnical Commission (IEC), ASTM D3638, DIN 53480, BSI 5901, VDE 0303-TEiL1, AFNORC 26220 have developed a standardized test described as a comparative tracking index test (hereinafter referred to as the "CTI test"). Under the CTI test, a pair of standardized electrodes are disposed on a surface of a test sample of the dielectric material of an insulator and an electrolyte is deposited on the dielectric between the electrodes by a slow dripping process.
Failure of a dielectric specimen under the CTI test is defined by a current of at least 0.5 ampere for two seconds before at least 50 droplets have fallen onto the sample. The comparative measurement of failure is referred to as the "comparative tracking index number", or simply "CTI", and is the magnitude of the highest voltage reached before failure.
To improve accuracy of the CTI test, IEC, ASTM, BSI, etc. test Publications specify a number of test parameters. First, the electrodes are to be mutually separated by a distance of 4.0 (+0.1, -0.1) mm during testing. The droplets striking the test sample are to be of a diameter of 20 (+3, -0) mm.sup.3 and are to be released from a specified height above the test sample. The test procedures recommend cleaning the electrodes before each use. Further, the electrodes must have a specified weight such that the electrodes exert a constant force against the surface of the specimen.
While the CTI tests have been of significant value in providing comparative data on dielectric materials, the results of the tests often vary significantly for identical specimens. Such variance creates uncertainty in the validity of any particular test result and offers the opportunity for abuse of the testing procedure.
One invention intended to reduce the variability of the IEC test was provided by Saito et al. in U.S. Pat. No. 4,339,708 (Saito). Saito recognized that some of the variables associated with testing may result from contamination of the droplet nozzle, and went on to introduce other variables by allowing movement of the droplet nozzle before and during release of the droplets. Such movement, while intending to reduce variability of the IEC test, may actually have increased variability.
The CTI test is of considerable importance not only to power companies but also to manufacturers of any power consuming device subject to environmental factors. Because of the importance of CTI testing a more reproducible method is needed for implementing the test of IEC Publication 112.